Solid ammonium thiosulphate



Ott. 22, 1940. w, H, Hlm; 2,219,258

SOLID AMMNIUM THIQSULPHATE Filed D80. 5,' 1959 CoA/CEN TPA rip 1NVENTOR.

ATTORNEY.

Patented Oct. 22, 1940 UNITED STATES. PATENT OFFICE Y 2,219,258 SOLIDAIVHVIONIUM THIOSULPHATE William H. Hill, Mount Lebanon, Pa., assignor to American Cyanamid Company, New York, N. Y., a corporation of Maine Application Decembers, 1939, serial No. 307,600 claims. (o1. 23-115) The present invention relates to the production process will rst be described regardless of the of solid ammonium thiosulphate from its aqueous source of the salt solution. o so1utions, It has been discovered that if ammonia, either Ammonium thiosulphate, while reasonably stain the form of gas or a liquid, is Aadded to an ble as such, undergoes decomposition when heated aqueous solution of ammonium thiosulphate in to any great extent, such as when its recovery such a way that ammonia is dissolved therein, is attempted from aqueous solutions by removing crystals oi ammonium thiosulphate will be pre- Water through evaporation. For this reason, it cipitated by reason lof the fact thatat a proper is dill'icult, if not impossible, to obtain ammonium concentration, ammonium thiosulphate is comic thiosulphate in the pure state by heat evaporapletely insoluble in aqueous ammonia solution.

tion from water solution. Under these circum- Consequently, an aqueous solution of ammonium lo stances, and in order to prevent this thermal thiosulphate may be partially or completely decomposition, it becomes necessary to resort to stripped of the salt by adding ammonia thereto. vacuum concentration, which involves expensive This may be accomplished either at atmospheric iff-i' apparatus and careful control. pressures or pressures greater than atmospheric. 15

The principal object lof the present invention, The fact that ammonium thiosulphate is very therefore, resides in the recovery of solid arn soluble in water solution and is completely inmonium thiosulphate from aqueous.` solutions soluble in water containing appropriate quantithereof by methods not-involving the use of heat ties of dissolved ammonia makes it possible, at al1, but, on the contrary, depends upon the through the aDpmpTate adjustment of` the amso principle of lowering the solubility of the salt monia content ofthe solution, to obtain very line in its solution by adding ammonia thereto. or very large crystals 0f the salt 01' Perhaps What Another object of the invention resides in the is more important, Crystals 0f any size, but unproduction of crystals of ammonium thiosulphate form in dimension.

in anhydrous form in a good state of purity. This For instance, assuming an aqueous solution of g5 is a corollary of the object above mentioned in ammonium thiosulphate,.the salt being less soluthat by using a proper concentration of ammonia ble in ammonia solution than in water, ammonia in the aqueous solution of ammonium thiosulmay be added until the salt will begin to be phatethe sali-,may be forced to crystallize therethrown out of solution by reason of the fact that from Without water of crystallization. This is of the latter is supersaturated therewith. By confm advantage in that it reduces the total weight of tinuing the ammonia addition, the solution may the salt during shipment. be completely freed of ammonium thiosulphate l Another important object of the invention releaving behind only ammonia dissolved in water. sides in the productionof solid ammonium thio- If, on the other hand,'the ammonia is added sulphate in a good state of purity without resortto the ammonium thiosulphate solution slowly so 35 ing to heat wherein a completely cyclic and conas to substantially maintain the solution in the tinuous process results by reason of the fact that metastable zone, that is, Where crystal growth the nal liquor from which the ammonium thiocan take place but Where no new crystals result, A sulphate has been completely removed may be large and uniformly sized crystals of ammonium recycled as such to the start of the process for thiosulphate maybe Obtained. This is based on 40 the synthesis of additional quantities of the salt. the discovery that there is a range in concentra- Some ammonia may be removed from the final tion of ammonia in the salt solution at which time liquor before recycling so as to have at the start the solution is saturated with the salt. This is a s, of ythe process such a concentration of ammonia Inetastable ZOne and in which Crystals will not as will yield with sulphur dioxide and sulphur an be precipitated. If the ammonia concentration amount of ammonium thiosulphate most desired iS built up above thsmaximum, Crystal nuclei in the process, will form because the solution has reached a state The invention further consists in the novel 0f supersatllraton' If n0 Inore ammonia is steps and combination of steps more fully hereinadded, Crystal 11110161 Wlll COHtlnlle t0 fOlIn until after described and shown in the drawing, which the solution drops back mto the metastable zone 50 constitutes a ow sheet of a preferred process. or Zone 0f saturation. At that point, no new inasmuch as the principal object of the invencrystals will form. While in this phase, however, tion resides in lthe recovery of crystals of amammonium thiosulphate can and will be thrown monium thiosulphate from aqueous solutions out of solution to build up upon the crystal nuclei thereof without the aid of heat, this part of the formed, to produce crystal growth. This process ,55 i

will continue until the solution reaches the saturated phase at which time crystal growth will stop. If, therefore, the ammonia addition is continued at such a rate as not to exceed the rate of crystal growth, that is, to maintain the solution in a state of metastability, the crystals already formed will continue to grow to any desired size.

Where lne crystals are desired, ammonia may be added at a rate greater than that at which crystal growth may take place, that is, to maintain the concentration of ammonia in the solution such that the solution will always be in a state of supersaturation. Consequently, the ammo.- nium thiosulphate may be completely removed. from the solution, leaving only a concentrated solution of ammonia. This will Aproduce very line crystals. y

Where crystals of various sizes are to be produced, it will be expedient to periodically increase the ammonia content of the solution to such an extent that supersaturation is reached for a While, whereby crystal nuclei are. formed. If, then, the ammonia. input is stopped,.crystal growth will take place. on the nuclei already formed.V Additional quantities of ammoniasuicient to again enter the Zone of supersaturation will cause an additional group of small nuclei. By repeating the process, when the solution again reaches the stage of saturation or metastibility, crystal growth will take place not only upon the second crop of small crystals but upon those formed originally which have grown. In this way, any number. of sizes of crystals may be obtained at Will.

In the. rst instance, where crystals of desirably large `size `are to. be produced, it will be noted that uponremovalof these large crystals, the solution still contains ammonium thiosulphate. The liquor may be kmade barren of the salt by continued addition of. ammonia until all of the ammonium thiosulphate has been thrown out.

Such a process. produces the saltin crystal form in a very pure state by reason'. of the fact that no heat has been .applied to the solution and, consequently, there is no chance for thermal decomposition. Moreover, under the above conditions,. ammonium thiosulphate is obtained as anhydrouscrystals.. i

Y Referring. nowV to av complete process, reference is made to the flow sheet which indicates a preferred` method of ammoniumthiosulphate synthesis resulting in anaqueous solution of the salt from which the latter may be obtained from the ammonia method above described.

It is known that if. sulphur dioxide isreacted with ammonia, the ammonium sulphite is formed. This may be done in the gas phase or inaqueous solution. I n the latter case, depending upon the initial concentration of the ammonia in the aqueous solution, a solutionr of ammonium sulphite or a suspension thereof may result. On contacting such solution or suspensionk with sulphur, conversion to thiosulphate takes place. Although this conversion 'takes place at room temperature, it is usually carried out at temperatures of about 509 to 100 C., merely for the greater speed of reaction. I prefer a temperature of about 50 C. It is desirable to so regulate the initial ammonia concentration in the Water that on. conversion of the first formed ammonium sulphite to thiosulphate," asolution of about 30% of the latter results because it has been found by experience that when it is attempted to produce higher concentrations, the. lastiamounts of ammonium sulphite are only very slowly converted and, therefore, the process becomes unduly sluggish.

The resulting ammonium thiosulphate solution may be ridded of its solids content by precipitation with ammonia, as above described. This may be done in several ways dependent upon what temperature was used in the conversion of ammonium sulphite to ammonium thiosulphate. If an elevated temperature was used, the ammonia can either be forced into it under pressure either as a gas or as a liquid, until a sufficiently high. concentration is reached to precipitate it. On the other hand, the warm solution can be cooled downl and ammonia then introduced whereby higher pressures can be dispensed with. TheA end effect is the same inasmuch as a certain `ammonia concentration is needed regardless of temperature.

By ridding the solution of crystals of ammonium thiosulphate@` by L the. ammonia method above; described, there remains' at. the end. of. the.

process an aqueous solution of ammonia.. This liquor may then be returned to the: start. of the cycle for the formation'of additional quantities of. ammonium sulphite through introduction, of

sulphur dioxide',.and` the Vcycle repeated. They abovev cycle is .particularly` advantageous in that no. substantial quantities `of ammonia are lost nor is it necessary' tog deal `with compressors or thelike. z

While the invention hastbeen shown` and. de-

scribed with particular reference to specific em-l bodiments, it is to be.l understood that it is notte, be limited thereto but is to beconstrued broadly and restricted solely by thescope of the appended claiins.A

I claim:

l. A method, of recovering crystals. of ammonium thiosulphate from aqueoussolutions thereof which includes'th'e: step of introducing ammonia into the solution until crystals ofammonium thiosulphate are precipitated therefrom.

2. The method of. claim l. in-Which the `ammonia is` added at such a rate as to cause the solution to reach a Ystate of supersaturation whereby crystals of the salt. are precipitated andA thereafter adjusting the ratel of ammonia addition .so that nownevv crystals are formedv but so as to cause. ammonium. thiosulphate in.v solution to be thrown out. and grow upon` the` crystal. nuclei already formed.

3. The method of claim 1L in which'. theiammonia addition` is continued until the solution. is barren of ammonium thiosulphate.

4. A method of obtaining crystals of ammonium thiosulphate which comprisesforming an aqueous solution of ammonium sulphite by adding sulphur dioxide toan aqueous' solution. of l ammonia, adding sulphur thereto. to. form an aqueous solution of ammonium thiosulphate, adding ammonia tothe ammonium thiosulphate solution until. the solution is supersaturated with theA ammonium thiosulphate, continuing the. ammonia addition. until. thev liquor is barren of thiosulphate; removing the latter' and. recycling the resultant concentrated aqueous solution of am'- monia to the start of the cycle to'react with additional quantities of sulphur dioxide to form ammonium sulphite. p v

5. A method of obtaining crystals of ammonium thiosulphate which comprises forming an aqueous solution `of ammonium sulphite by adding sulphur. dioxide to an aqueous solution of ammonia, addingsulphurtheretotozfornr an aque- (5 of until the ammonia content of this solution is such as will produce a. 30% solution of ammonium thiosulphate after reaction with appropriate quantities of sulphur dioxide and sulphur, andv returning this aqueous solution of ammonia to 5 the start of the cycle. y

WILLIAM H. HILL. 

